Audiophile Review!Pass Labs HPA-1Headphone Amplifier

Right out of the box, it was such a pleasure to review the new Pass Labs HPA-1 Class A MOSFET headphone amplifier. A long time coming, the Pass HP amp's audio character is akin to the “super class A” Pass XS amplifier design — with a generously open soundstage, rich, smooth transient response and deep-bass performance.

Couple its sonic output with its ability to drive any single-ended headphone, and you have one of the best headphone amps available — anywhere at any price. The fact that it is only $3,500 makes it a genuine bargain.

Features

The Pass Labs HPA-1 features a custom, low-noise toroidal power transformer, feeding a discrete, low-noise regulated power supply for the audio circuits. According to Pass, “the importance of the power supply is often overlooked (in designing audio products) but plays a large part in overall performance of the amplifier,” even at the lower power required for driving headphones

The HPA-1’s amplifier circuits are low-feedback, wide-bandwidth discreet designs employing a JFET input stage and a MOSFET output stage, biased into a Class A-biased, direct-coupled output stage. The HPA-1 can drive headphones from 15 to 600 Ohm loads, including the rising tide of planar magnetic headphones.

Everything Audio is totally sold on the Pass HPA-1. As a high-end, audiophile headphone amplifier/line preamp, it is a bargain. It takes headphone listening, especially with hi-res material and the newer ultra-accurate D/A converters, to a higher level.

This elegant, simple preamp exudes class with its machined casing, large volume control and simple chassis lines. Beautiful simplicity as I call it. The Pass HPA-1 is elegantly simple to operate as well. The brushed aluminum front panel includes one single-ended, 1/4-inch HP input, a large volume knob, and switches for two inputs. Around back are a set of variable unbalanced RCA output jacks for line-stage use, two sets of RCA inputs and the master power button. That is it.

Spec-wise, the HPA-1 distortion is rated at less than .005% (THD+Noise, 1V out); bandwidth is listed at 10 Hz to 100 kHz (-1 dB). Output power is rated at 3,500mW at 20 Ohms and and 200 MW at 300 Ohms. The overall gain is 8 dB. Input impedance is 250 Ohms and output impedance is 2 Ohms.

The HPA-1 amp measures 11 inches wide, 13-inches deep and 4.5-inches tall. It weighs about 14 pounds. There is no remote control. This is, indeed, a purist HP amp/preamp.

A new design

Pass Designer Jam Somasundram explained that the Pass Labs HPA design presented him with a conflicting set of requirements, as headphones have impedance's that vary between 15 and 600 ohms, and that Pass wanted the HP amp to also be used as a quality line preamp.

“To get around this obstacle,” he explained. “We had to design in high supply rails and have the ability to supply a fair amount of current. So the basic design follows a small power amplifier but has to meet the requirements of a preamp — with relatively low noise and distortion.”

I am impressed with what I hear with the Pass HPA-1. Amazingly life-like, detailed, open, musical soundstage with a smooth, easy-to-listen character, and it can drive any headphone.

Somasundram noted that to get the headphone amp to handle a wide range of headphone loads and to offer excellent performance as a high-quality line preamp, the HPA-1 was created from a brand new Pass Labs design that includes the aforementioned JFET input stage, bipolar current mirror and MOSFET output stage — operating in the high-current and high-voltage drive path.

To succeed with his design, Somasundram said the power supply was key. “We had to develop a custom transformer that incorporated a Faraday shield and Mu metal band around the circumference to meet noise requirements,” he added. “It is rated for more than three times of what is needed by the circuit. It took us seven prototypes to get to the final design.”

The HPA-1 power supply has in excess of 40,000uF of capacitance and uses a discreet regulator. The only place that an integrated circuit was used in the design was the servo that controls the offset, which is not directly in the signal path, according to Somasundram.

The HPA-1 is controlled by a custom-programmed processor. The unit mutes itself for 20 seconds before turning on (the LED flashes during the mute state), then the LED stops flashing and stays on. In the event of power interruption, the unit will go through the mute cycle on resumption of power (to avoid any thump or clicks), then return to the settings in place before the loss of power.

Somasundram said he was not obsessed with specs for the HPA-1 design, but was focused on the ultimate listening enjoyment and the ability to handle any type of headphone that audiophiles wanted to use. The design testing was primarily done with simple test equipment and Pass engineer listening panels.

Somasundram noted: “We only did formal noise and distortion tests after the design was complete and, in this case, both the subjective and measured results were in agreement. The danger in designing audio equipment is trusting your test equipment too much, while trying to obtain lower distortion and noise numbers.”

Pass President Desmond Harrington emphasized that the HPA-1 turned out to be a really good-sounding headphone amp that is not that expensive. I heartily agree and could tell, in short order, what all the buzz was about surrounding this product.

Source players included an Oppo BDP-105, modified with a discrete analog output path, courtesy of Bill Thalmann at Music Technology in Springfield, Va.; a Macbook Pro computer and a Dell Venue 8/USB Audio Player Pro Android hi-res player in “bit perfect" mode. The computers were routed to various DACs via USB, whose outputs were connected to the HPA-1.

Although it is a Class A amplifier, the Pass HPA-1's thick, aluminum housing takes a long time to feel warm, and I did not want ti listen until it was “warmed up.” Its mostly a mental thing, I like my Class A and tube gear to feel like its working warm and toasty, so I never did any listening through the HPA-1 — until at least 40 minutes after I turned it on.

My first demo tune was “The So Real” tracks from the Warren Bernhardt — So Real Jazz SACD from 2001, recorded by Tom Jung for DMP. I have played this track for reference listening hundreds of times, and I know it inside and out. The drum cymbals, and drum stick-on-snare is incredibly real sounding (pardon the pun), with a ultra-accurate brush-on-cymbal sheen and upper-register Steinway piano.

A properly, electronics rendered play of this track is awe-inducing with how much space you hear. Lesser gear diminishes the space, but Pass HPA-1 brings one of the deepest, widest HP listening impression I have ever heard on this track. But the musical color is not thin; in fact, it is rich and warm, like a really good tube amplifier. Although its design differs from the Pass’ super Class A Series XS amplifiers, the sonic result is very similar. A huge, but not exaggerated soundstage, with full bass lines and that “just right” harmonic smoothness.

Key to HPA-1's sound: a hefty power supply

That character follows the preamp out line stage as well. Coupling it with the Pass X350.8 MOSFET stereo amplifier, the amp reinforced the warmth and expanded the impressive stereo image out through the MartinLogan Montis electrostatics. Even on a solid-state, bipolar amplifier, such as the old standby Bryston 14B-SST II, the soundstage impression came through.

Back to the headphones, I switched the music genre to Classical, and played the 2L Mozart Violin Concerto In D from my Macbook Pro and Audirvana Plus hi-res player. The music was routed through the Bryston BDA-3 DAC/Pass HP amp tandem. The DXD 24/352 recording is slight dry, but contains an extremely detailed violin tone — with full bow-to-string harmonics and an immense dynamic range when the orchestra kicks in.

A definite award winner

Through the HPA-1 and AKG K702s, plus the Shure SRH-1840 HPs, the detail and dynamic range are intact, but some of the violin’s rough edges are tamed (compared to the outboard DAC headphone amps on the Oppo HA-1 and Benchmark DAC2. without sounding muted. It added just a tinge of audible golden glow to the violin. Again, it reminds me of the best of a tube amp stage.

Comparing all the built-in HP stages in my DACs, they all exhibited similar, in a general sense, sonic signature with varying degrees of refinement. The Oppo, Benchmark, etc. sound very open, detailed and dynamic, but none of them have that wide-open glow of the Pass. Besides the good stuff, I even found myself listening to harder-edged sounding CDs and higher rate MP3s through the HPA-1.

Conventional or planar magnetic

I switched off to planar magnetic-driver Oppo PM-1 headphones, for a sampling of the Allman Brothers "Blue Sky" track — from the Eat a Peach SACD. This is quite a good 1970's’analog recording — with the clearly delineated acoustic and electric guitar layers in a nice, open mix. Via the Pass, its sound is a bit wider with an easier-to-hear acoustic rhythm guitar permeating the the track’s play. And I love the dual-electric guitar lead interplay in the long solo, On most typical headphone amps, it sounds pretty hi-res, but through the Pass, it seems to have more life.

After about 50 various plays of different kinds of music, I came to the conclusion that the Pass HPA-1 does not sound bad on anything. The 24/96 remaster of Led Zeppelin IV’s “Stairway To Heaven?" No problem. John Paul’s Jones prominent bass line did not seem to be dragged down by the warmth of HPA-1 at all.

After about 50 various plays of different kinds of music, I came to the conclusion that the Pass HPA-1 does not sound bad on anything. The 24/96 remaster of Led Zeppelin IV’s “Stairway To Heaven?" No problem. John Paul’s Jones prominent bass lines did not seem to be dragged down at all by the extra warmth of HPA-1.

How about LP records? That was no problem for the Pass HP amp either. I played the original pressing of TheIsao Suzuki Quartet — Blow Up, an audiophile LP from 43 years ago, using my Clear Audio Emotion, a Benz Wood cartridge and the Rogue Audio’s RP-5 tube phono stage. Wow did Mr. Suzuki’s bowed bass cello sound impressive through the headphones and via the line stage/amp/speakers — with its a big, warm, aggressive cello bow cadence, punctuated by the ultra dynamic drum kit.

The Pass sound is music

In recent years, Pass Labs has focused its gear designs on making music listening enjoyable with out obsessing on specs. If it has a little warmth or texture that makes audiophiles more enthusiastic about listening to music, they are happy. The XS series leans that way, so do the .8’s and now the HPA-1. In the end, listening to music that satisfies is what matters most. Do you like what you are hearing?

Love that big volume control

I am deeply impressed with what I hear with from the Pass HPA-1. Amazing lifelike, detailed, open, musical soundstage with a smooth, easy-to-listen character, It can drive any headphone, including my AKG’s, which often force me to push many headphone amp knobs way up the scale to get them to an average level.

No complaints with the HPA-1. Some may notice the lack of a remote control, but I am a knob twirler and button pusher. The lack of a remote does not bother me. I go to the rack all the time and turn up the volume (or to turn it down), so that big knob suites me just fine. The other omission is a lack of balanced circuit, but Pass engineers are working on a balanced version for those who preferred balanced headphones. Stay tuned for any news on a balanced version.

The verdict

Considering its $3,500 price and wonderful sonic persona, I am totally sold on the Pass HPA-1. As a high-end, audiophile headphone amplifier/line preamp, it is a bargain. It takes headphone listening, especially with hi-res material and the newer ultra-accurate D/A converters, to a higher level. The HPA-1 is a no-brainer for an Everything Audio NetworkStellar Sound Award, and our only choice for EAN Product Of The Year in the Headphone Amp category.